I am making a model of half of a PEM fuel cell. I have onhe inlet of hydrogen and two outlets. Outlet_1 is at a set pressure (standard gas outlet for the fuel cell) and outlet_2 is acting as the catalyst layer (i.e. where hydrogen reacts and crosses through the membrane).

I think you can make an INFORM statement which will calculate the quantity you need (density*area*v?). Check out the INFORM documentation for the syntax. Then just look at the calculated values in the grid cells right *next* to the outlet.

There is probably a more elegant way to do this, but this is what springs to my mind.